1. Field of the Invention
The present invention relates to a tenter device used in a film production apparatus for solution casting method.
2. Description Related to the Prior Art
To produce a polymer film having no unevenness of thickness and optical properties, a tenter device has clips for holding lateral edges of the film, and moves the clips which hold the film. The film is dried while being transported. The clip is assembled such that a flapper is rotatably attached to a U-shaped frame through an attaching shaft and dry bearings. The flapper is biased to a holding position by own weight. When the flapper is at the holding position, the film is held between a lower end surface of the flapper and a film holding surface of the frame. In addition, when an upper end surface of the flapper contacts a clip opener, the flapper is rotated to a retreat position where the holding of the film is released. The clip starts the holding of the film at an entrance of the tenter device, and releases the holding at an exit of the tenter device. Tenter devices using this type of clips are disclosed in Japanese Patent Laid-Open Publications No.11-77719 and No.11-90943.
However, when this conventional tenter device is used for producing cellulose acylate film, although at the beginning a film having high retardation value and superior optical properties can be produced, after a certain period of time, some clips start misholding of the film because of poor smoothness of the swing of the flapper. When a part of the film is misheld by the clip, there occurs unevenness of thickness or optical properties of the dried film.
In addition, there becomes a possibility to break the soft film formed by solution casting, when springs or the like are used for biasing the swing of the flapper so that the flapper is swung to the holding position regardless of lack of the smoothness of swing.